You'll need to debug to get it right but you should be able to clean up the list here.

Step 2: Fix your FilterExpression

After you make your filter values pretty, you'll notice that the filter won't work anymore. This will be due to your FilterExpression trying to filter on the new "pretty" values instead of the ugly String;#filterexp1 values which are the actual values.

You probably want to override OnPreRender in your web part to dynamically fix your DataSource's FilterExpression: